Penguins are bringing forward their breeding season due to warming temperatures

Every year, penguins are returning earlier to the nesting grounds where they lay their eggs and raise their chicks. This isn’t a phenomenon limited to a single colony; it has been observed in dozens of colonies and across three different species. On average, they have returned two weeks earlier, but in some cases, by almost a month. This places these birds among the living beings that have most significantly altered one of life’s fundamental patterns: raising the next generation. The authors of the research, published January 19 in the Journal of Animal Ecology, relied on the deployment of dozens of cameras with built-in thermometers. This allowed them to identify a clue to what is happening: an accelerated increase in temperature. What they don’t yet know is whether the penguins are adapting to or being overwhelmed by climate change.

All penguin species, except for the emperor penguin, which nests on the ice, need to lay their eggs on land, in an area free of snow and as dry and warm as possible. In fact, researchers link their massive nesting efforts to the melting ice: their droppings, in addition to the heat they absorb, are rich in salts and very dark, trapping more radiation, which accelerates the melting of the ice. Furthermore, the 18 cataloged species tend to return to the places where they last bred. And all of them are also considered sentinels of the health of Antarctic ecosystems. Ignacio Juárez, a researcher at the University of Oxford and lead author of the study, emphasizes this point: “Studying the oceans is a very difficult task, and in Antarctica, logistically impossible due to the amount of ice. Therefore, we rely on studying species that, like penguins, spend their lives at sea but return to land to reproduce.”

Juárez travels to Antarctica every austral spring or summer to study penguins. In a project initiated in 2011, his team devised a system of cameras that take images at set intervals (every hour, every five hours, every 24 hours, etc.). In total, they placed 77 camera traps in 37 colonies of three different species: Adélie, chinstrap, and gentoo penguins. The network covers virtually the entire geographic range of these birds. Each image is accompanied by the temperature at the time it was taken. “There will be many studies like this, but this is the first deployment of its kind in 15 years,” Juárez points out.

The results are conclusive. The first to arrive at their nesting and breeding grounds are the Adélie penguins (Pygoscelis adeliae). On average, they now gather around October 15, having moved their arrival forward by one day each year. Throughout their geographic range, the trend is the same, although the specific day varies. For example, the three colonies on Signy Island, part of the South Orkney Islands archipelago, arrive on different days, but all three have arrived earlier. The second species to arrive is the chinstrap penguin (Pygoscelis antarcticus). They now arrive around October 20. All their colonies do so about two weeks earlier. The third is the gentoo penguin (Pygoscelis papua), which arrives at its nesting areas around November 1, an average of 16 days earlier since the camera traps were deployed. But some of its populations do so 24 days earlier.

“They reuse last year’s nests with minimal changes,” Juárez recalls in an interview. “Once you see them in the nest and they’re no longer moving, that’s the day we say the season has begun. In fact, we observe that as this settling-in day comes earlier, all the other phenological milestones of their life cycle also come earlier: egg-laying, hatching, raising the chicks…” the Spanish researcher explains. Penguins gather in colonies that can range from a few dozen to thousands of individuals. This strategy offers multiple benefits, such as shelter from the cold and, above all, the security that numbers provide against predators, which on land are only other birds, like petrels and skuas, that feed on the chicks and eggs.

Each image also recorded the temperature at the time it was taken. This has allowed researchers to link the early arrival to one of the manifestations of climate change: warming. Even in August, in the middle of the austral winter, temperatures are rising more each year. In October and November, the months of the mass arrival, the increase reaches 0.41°C per year. Previous studies have already shown that warming is more pronounced at the poles, but in the colonies it is even greater, up to four times higher than in the rest of Antarctica.

Although all indications point to rising temperatures as key to this earlier onset of the most critical period — breeding — it remains unclear whether these changes reflect an adaptive response, which could create a mismatch with other ecological factors. Among the consequences of warming that are altering the Antarctic ecosystem is the loss of sea ice extending kilometers beyond the continent. In principle, this would help penguins reach their nesting grounds. However, the accelerated melting is interfering with the process that begins with the annual bloom, the emergence of vast expanses of microalgae nourished by particles trapped in the ice and stimulated by sunlight. This process continues with krill feeding on the algae, then with penguins relying on this crustacean as a food source, and finally with orcas and leopard seals that depend on the birds for survival.

“Given that penguins are considered an indicator of climate change, the results of this study have implications for species across the globe,” emphasizes Fiona Jones, co-author of the research and also a researcher at Oxford. The earlier start to their breeding season is, according to the authors, the largest ever recorded among birds and one of the most extreme events recorded among living things, whether animals or plants. “We need further monitoring to understand whether this record-breaking advance in the breeding season for these penguin species is affecting their reproductive success,” Jones concludes.

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